1. Technical Field
The present disclosure relates to a method of manufacturing an image sensor for converting optical images to electric signals, and more particularly, to a method of manufacturing an image sensor having a reduced dark current in a photodiode.
2. Discussion of the Related Art
A photodiode is used for an image sensor to convert incident light to charges. Image sensors such as, for example, a complimentary metal oxide semiconductor (CMOS) image sensor (CIS) and a charge coupled device (CCD) image sensor are widely used. In the CIS and the CCD image sensors, incident light on a two-dimensionally disposed photodiode is converted to signal charges (i.e., electrons) which are sequentially read by signal voltages in accordance with a time axis. The CIS and CCD image sensor can have similar structural configurations. In the CIS image sensor, charges are converted to voltages in a plurality of unit pixels, and signals are output by a switching operation in a signal line. In the CCD image sensor, signal charges are transferred by the order of a vertical register and a horizontal register, and the signal charges are converted to voltages before an output terminal.
In conventional image sensors, a charge transfer efficiency and a charge accumulation capability are decreased due to noise or dark current, thereby causing image defects. The dark current refers to charges stored in a photosensitive element of the image sensor without an input of light. The dark current can be caused by defects existing at an interface portion between an isolation layer around a photodiode and an active region. The dark current can also be caused by a silicon dangling bond. The silicon dangling bond is located, for example, around side-portions of the photodiode or adjacent to a silicon substrate surface or an isolation layer, and may easily generate charges without the input of light. Hence, dangling bonds existing around side-portions of the photodiode may generate signal charges even if light is not incident, thereby causing a dark current. Since the dark current is generated due to a crystal combination, an amount of the generated dark current is varied in accordance with each photodiode. Thus, when an image sensor reacts due to the dark current, the quality of image in a display device using the image sensor can be deteriorated.
A conventional technology for minimizing the dark current generated around the side-portions of the photodiode forms a hole accumulation region. The hole accumulation region can be formed by implanting p-type impurity ions into an active region formed between an isolation layer and a photodiode using a photoresist pattern formed by a photolithography process using an ion implantation mask. However, in the conventional method, if a substrate area opened by the photoresist pattern is increased, a photodiode region and an active region formed between an isolation layer and a photodiode are also opened. Thus, an effective area of the photodiode and a saturated current are reduced, thereby causing a deterioration of photosensitivity. The deterioration of photosensitivity can be prevented by minimizing the substrate area opened by the photoresist pattern. The conventional method using the photolithography process causes misalignment of a photoresist pattern or causes a critical dimension (CD) deviation on a wafer. The conventional method using the photolithography process has a difficulty in forming a photoresist pattern having a vertical profile. Thus, there is a need to ensure both an effective area of the photodiode and a sufficient process margin during the photolithography process by minimizing the substrate area opened by the photoresist pattern.